Does Bitcoin-QT use more CPU when its idle than during synchronization?: I am running the standard client on a m...

I am running the standard client on a micro Amazon EC2 instance. When I am synchronising Bitcoin with the network, it uses about 50% of the available CPU. After it is fully synchronised, I leave it alone and it cranks up to 100% by itself. What can be causing the standard client to behave in this manner? Is it due to possibly high transaction volume, sharing information with peers, or perhaps something else?